As discussed previously, external validity refers to the extent to which the
results of the study can be generalized. That is, will the same (or similar)
results be obtained with other populations, conditions, experimenters, other
measurements, and so forth? When research fi ndings can be generalized outside
the confi nes of the specifi c study, the research is said to have external
validity. Any characteristic of the study that limits the generality of the
results is a threat to external validity. Some of the more common threats to
external validity follow, grouped into three major categories.
Category 1: Generalizing Across Participants or Subjects
The results of a study are demonstrated with a particular group of individuals.
One question of external validity is, “To what extent can research results
be generalized to individuals who differ from those who actually participated
in the study?”
1. Selection bias: In Chapter 5 we defined a biased sample as one that has
characteristics that are noticeably different from those of the population. A